1. Field of the Invention
The present invention relates to a sintered multi-layer ceramic body and a method for making the same and in particular, to a sintered multi-layer ceramic body which can assure high dimensional accuracy and high reliability and a method for making the same.
2. Description of Prior Art
Multi-layer technology for ceramics has become essential especially in the fields of electronic industries. For example, mention may be made of techniques such as miniaturization or enhancement of performance of electronic ceramic devices such as multi-layered ceramic capacitors and piezoelectric devices and application to ceramic multi-layer substrates which are essential for high-density packaging of semiconductor devices. These are generally produced by a green sheet method and are known as sintered multi-layer ceramic bodies.
However, ceramics generally undergo a large dimensional change (shrinkage) when sintered, which is apt to be affected by variation of starting materials or processes. Therefore, keeping the dimensional accuracy when sintered at a high level is very difficult. Furthermore, the high dimensional accuracy is also required for a sintered multi-layer ceramic body and especially when the sintered body is used as a multi-layer substrate, a high positional accuracy (surface dimensional accuracy) of wiring formed on the surface of substrate is required since semiconductor chips must be mounted thereon and connection with the chips must be formed. Furthermore, these ceramic substrates are subsequently subjected to operations such as plating, attaching of pins and formation of a thin film layer, and mechanical reliability in these operations is also an important requirement.
Furthermore, when thin film multi-layer wirings must be formed on these ceramic substrates in later steps, it is also an important requirement that the ceramic substrate can be set on a device for the production of thin film circuits with high accuracy. In such case, since the substrate is set based on the periphery of the substrate, it is necessary that the external size (maximum size) and peripheral shape of the sintered body are high in accuracy.
In general, a multi-layer ceramic laminate is fired without application of load (pressureless firing). This has merits of simplicity and low costs, but it cannot avoid uneven shrinkage upon firing and it is very difficult to stabilize surface dimensional accuracy to a large extent. Especially when the laminate has complicated conductor layers therein, it is difficult to assure the dimensional accuracy and besides, occurrences of warpage, peeling and blister are severe problems.
For these problems, Japanese Patent Kokai Nos. 57-32657, 62-5848 and 63-31754 have proposed methods of applying a small load of 0.5-20 g/cm.sup.2 to the laminate in firing thereof. According to these methods, warpage, peeling and blister of the laminate can be diminished without hindering the firing shrinkage of the laminate, but uneven firing shrinkage cannot be simultaneously diminished. Therefore, these methods cannot assure the high dimensional accuracy.
Japanese Patent Kokai Nos. 60-137884 and 1-225546 disclose methods of sintering the ceramic laminate under application of a load of 25-200 kg/cm.sup.2 for inhibition of shrinkage at the sintering. However, according to these methods, a frame must be used in firing in order to sinter the laminate without causing substantially any change in surface dimension. If the laminate is sintered with its side faces being free without using the frame, in fact, the sintered body is collapsed owing to the very large load and the accuracy of the surface and external dimensions of the sintered body cannot be ensured (its shape cannot be retained). On the other hand, when firing is carried out using the frame, the sintered body is reacted with the frame material and adheres to the frame, resulting in the side faces of irregular shapes or the frame material bonds firmly to the sintered body which necessitates grinding to remove the frame material or there occur severe problems in costs and operability because the frame must be thrown away after use.
On the other hand, Japanese Patent Kokai No. 62-260777 discloses a method of sintering the laminate under application of a medium pressure between those employed in the above two methods, to thereby avoid shrinkage in XY plane, upward warpage and distortion. This method can inhibit sintering shrinkage in the XY plane of substrate, but makes no mention of the shape of its side faces and means for controlling the shape.
In the above-mentioned conventional methods, the reduction of dimensional accuracy in the surface layer caused by uneven firing shrinkage cannot be avoided when no pressure or a relatively low pressure is applied. A further problem is that ideally the side faces must be in the same flat state as before firing, but actually the uneven shrinkage occurs due to inner wiring or the like, resulting in irregular side faces.
When a large load is applied, the laminate is considerably collapsed during firing, or the laminate reacts with and bonds to the frame used for prevention of the collapse and in either case it is difficult to control the external dimensional accuracy of the sintered body.